Optical constants of polycrystalline CuGaTe2 films

October 1995

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Optical Materials 4 (1995) 787-790

Optical constants of polycrystalline CuGaTe2 films M. Sesha Reddy, K.T. Ramakrishna Reddy, B.S. Naidu, P.J. Reddy Department of Physics, Sri Venkateswara Universi~, Tirupati-517 502, India Received 16 December 1994; accepted 13 July 1995

Abstract

Thin films of CuGaTe2 with thicknesses in the range, 0.1-1.0/xm were deposited on Coming 7059 glass substrates by flash evaporation. The substrate temperatures, T~, were maintained in the range 373-623 K. The transmittance of the films was recorded in the wavelength range 900--2500 rim. The dependence of the optical band gap, Eg, on substrate temperature showed that the value of Eg varied from 1.21 eV to 1.24 eV. The variation of refractive index and extinction coefficient with photon energy was studied from which the material properties such as the limiting value of dielectric constant, e~, plasma frequency, wo, and hole effective mass, mff, were evaluated as e~ = 7.59, O)p 1.47 X 1014 and mff = 1.25 mo. =

In recent years interesting reports have been published on the preparation and characterisation of the ternaries and quaternaries of chalcopyrite semiconductors, particularly on telluride compounds [ 1-5 ]. However, the reported data on the opto-electronic application of the thin films of these compounds are meagre. We have taken a systematic study on the preparation and characterisation of flash evaporated CuzTe and CuGaTes thin films and some of the results have already been reported [ 6-8 ]. In order to use these films effectively for opto-electronic device applications, the measurement of optical constants, the refractive index, n, and the extinction coefficient, k, are very important. The refractive index which depends on the structure of the material and its composition varies with the wavelength of the incident radiation. This can be conveniently measured using the transmittance data of the films. The variations of n and k with the photon energy, h u were used to evaluate the plasma frequency of CuGaTe2 thin films effectively for the first time at which the reflectivity of the semiconductor tends to unity. In the process of evaluation the material para-

0925-3467/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSD10925-3467(95) 0 0 0 3 9 - 9

meters like dielectric constant and effective mass of charge carrier were also calculated. Thin films of CuGaTe2 were prepared by flash evaporation of the pre-reacted material prepared from its constiuent elements of 5N purity. The films were deposited at substrate temperatures, T~, in the range 373-623 K in a vacuum of 2 X 10- 6 Torr using a BALZERS 510E high vacuum coating unit. The deposition rate was maintained at 1.5 n m / s using a quartz crystal thickness monitor and the films were prepared in the thickness range 0.1-1.0 Ixm. The composition was studied using EDAX and the crystallinity of the films was examined with an X-ray diffractometer. A Hitachi U:3400 spectrophotometer was used to evaluate the optical parameters from transmission spectra measured in the wavelength range 0.9 to 2.5 Ixm. The compositional analysis revealed that the films formed at Ts = 523-573 K were nearly stoichiometric and the calculated weight percentages were Cu = 16.25 wt%, G a = 17.90 wt% and Te = 65.85 wt% which are comparable to the starting material composition. The films formed at Ts < 523 K were polycrystalline with excess gallium and tellurium whereas excess copper

M.S. Reddy et al. /Optical Materials 4 (1995) 787-790

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Fig. 1. Transmission spectra of CuGaTe2 films formed at three different substrate temperatures (x-x, Ts= 598 K; x- - -x, Ts= 548 K; x.... x, T, = 498 K). and tellurium deficiency were observed in the films formed at Ts > 573 K. The XRD studies showed that the films formed at temperatures in the range, 523-573 K were polycrystalline and single phase exhibiting 8

6

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chaloopyrite structure while the films formed at T~ = 450-523 K and Ts > 573 K were polycrystalline with secondary phases due to variation in the composition of the films. The transmission spectra of some typical polycrystalline CuGaTe2 thin films of 0.8 Ixm thickness formed at various substrate temperatures are shown in Fig. 1. The absorption coefficient, cx was calculated from the transmittance data. Plots of ( c~ hv) 2 versus hv for the three films are shown in Fig. 2. It is seen from the figure that the optical band gap varies in the range 1.21-1.24 eV corresponding to the variation in the composition of the films. The refractive index, n, of the films at different wavelengths was calculated using the formula [ 9 ] n=

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1"2

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PHOTON ENERGY h-a(eV)

Fig. 2. Plot of ( o ~ h v ) 2 versus h v for CuGaTc2 thin films formed at Ts= 598 K ( x - x ) ; T s = 5 4 8 K ( O - O ) ; and T s = 4 9 8 K ( A - A ) .

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where N=(l+n2ub)/2+2n~ub[(Tmax--Tmin)/ (TmaxX Tmi,)]. Here n~ub= 1.5 and Tm~x and Tmin are the transmittance maxima and transmittance minima at a given wavelength. The extinction coefficient was calculated using the relation k = ct A/47r in the photon energy range, 0 . 6 1.4 eV. The variation of refractive index and extinction coefficient with photon energy, h v for CuGaTe2 films formed at different substrate temperatures are shown

M.S. Reddy et al. /Optical Materials 4 (1995) 787-790 3.25

789

length, Ap. When n 2 >> k2 and toT<< I, the dielectric constant can be expressed as [ 11 ]

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Fig. 3. Variation of refractive index, n, with photon energy, hu formed at various T~ values ( T~= 598 K (x-x); T~= 548 K ((3-O); and T,=498 K ( O - O ) . 1.00

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where e= is the limiting value of the high frequency dielectric constant. Fig. 5 shows the variation of el and to- 2 of CuGaTe2 film formed at T~ = 548 K. The values of top and e= determined from the slope and intercept of the linear portion of el versus to-2 plot were top = 1.47 × l014 and e= = 7.59. Using these values and the charge carrier density, N,, evaluated from Hall measurements ( 6 . 4 x 1019/cm3), the hole effective mass, mr~ was calculated from the relation [ 11 ] top2= (47rNae 2)/m~e~. In the present study mff = 1.25 mo (mo is the rest mass of the electron) which is in agreement with the reported value of 1.26 mo in Ref. [ 12]. In conclusion, the data on the optical constants of CuGaTe2 thin films was reported for the first time. The optical parameters viz., absortion coefficient, energy band gap, refractive index and extinction coefficient were calculated using the transmission data. The plasma frequency, dielectric constant and effective mass of the carriers were also evaluated. It was found that m~ = 1.25 mo, mo being the rest mass of the electron.

1-60

PHOTON ENERGY, h'~(eV)

Fig. 4. Dependence of extinction coefficient, k, on photon energy, h~, deposited at different substrate temperatures. T~= 598 K (x-x) ; T~= 548 K ( © - © ) ; and T~=498 K ( Q - O ) .

in Fig. 3 and Fig. 4. The refractive index increased slowly to reach a maximum value at around 1.2 eV and decreased further while the value of k decreased slowly, reached a minimum and then sharply increased. From the k measurements, the free carrier relaxation time, T, was determined using the relation T = e=o~/2nkto3 and it was found to be 2 . 5 6 × 10 -11 ixs. The dielectric constant, E is given by e = el +ie2 where el and e2 are the real and imaginary parts of the dielectric constant. They can be expressed in terms of n and k as el = n 2 - k 2 and e2 = 2nk [ 10]. The reflectivity of a semiconductor in the infrared region shows anomalous dispersion as the incident photon energy approaches the corresponding value of plasma wave-

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M.S. Reddy et aL / Optical Materials 4 (1995) 787-790

References

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[6] P. Shameer Babu, M. Sesha Reddy, B. Srinivasulu Naidu and P. Jayarama Reddy, B. Electrochem. 5 (1989) 541. [7] M. Sesha Reddy, K.T.R. Reddy, B.S. Naidu and P. Jayarama Reddy, Solid State Physics (India) 37C (1994) 269. [8] M. Sesha Reddy, K.T. Ramakrishna Reddy and P. Jayarama Reddy, Materials Letts., in press. [9] J.C. Manifacier, J. Gasiot and J.P. Fillard, J. Phys. E 9 (1976) 1002. [ 10] J.I. Pankove, Optical process in semiconductors (Prentice Hall, Englewood Cliffs, New Jersey, 1971 ) p. 92. [ 11 ] H.A. Lyder, Phys. Rev. 134 A (1964) 1106. [ 12] K.V. Reddy and J.L. Annapurna, Pramana- J. Phys. 26 (1986) 269.